Method for manufacturing a metal powder, a metal powder, an electroconductive paste using the same, and a multilayer ceramic electronic component using the same

ABSTRACT

Provided is a method for manufacturing a metal powder by providing a reducing solution by dispersing caustic alkali, and hydrazine and/or hydrazine hydrate into a solvent; providing a metal salt solution comprising a salt of electroconductive metal, a rare earth metal salt and a solvent; and mixing the reducing solution with the metal salt solution to form a metal powder by depositing a hydroxide derived from the rare earth metal salt and by reducing the salt of electroconductive metal. With this metal powder manufacturing method, the sintering of the metal powder is restricted at a low temperature, the sintering initiation temperature is shifted to a higher level, and rapid sintering shrinkage is restricted, while ceramic grain growth is not accelerated.

RELATED APPLICATION

This is a divisional of U.S. application Ser. No. 09/841,199, now U.S.Pat. No. 6,503,291, filed Apr. 24, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metal powder and a method formanufacturing the same. More particularly, the present invention relatesto a method for manufacturing a metal powder for use in anelectroconductive paste suitable for forming internal electrodes of amultilayer ceramic electronic component.

2. Description of the Related Art

Conventionally, a multilayer ceramic electronic component, for example,a multilayer ceramic capacitor, comprises a ceramic laminate, internalelectrodes and external electrodes. The ceramic laminate is obtained,for example, by baking a raw ceramic laminate formed by laminating aplurality of raw (green) ceramic layers made of a dielectric material.The internal electrodes which are situated between the ceramic layers inthe ceramic laminate are formed by providing printing with anelectroconductive paste onto a plurality of raw ceramic layers followedby baking together with the raw ceramic laminate. One edge of each ofthe internal electrodes is exposed to one edge surface of theabove-described ceramic layers. The external electrodes are formed byapplying an electroconductive paste to the edge surfaces of the ceramiclaminate and then baking so as to connect to the edges of the internalelectrodes exposed at the edge surfaces of the ceramic laminate.

An electroconductive paste is used for such a multilayer ceramicelectronic component, and especially for forming internal electrodes ofa multilayer ceramic capacitor. The electroconductive paste contains ametal powder that functions as an electroconductive component. In recentyears, a base metal powder such as nickel powder and copper powder hascome to be used as the metal powder, in addition to a noble metal powdersuch as silver and palladium powders previously used. Furthermore, asprogress has been made in realizing miniaturization and thicknessreduction of the layers of a multilayer ceramic electronic component,finer particles are demanded for the metal powder contained in anelectroconductive paste for use in forming such internal electrodes.

As the particle size of a metal powder becomes smaller, the specificsurface area of the powder grows in proportion to the second power ofthe particle size, and as the specific surface area grows, the sinteringtemperature of the metal powder becomes lower. If internal electrodes ofa multilayer ceramic electronic component are formed with anelectroconductive paste comprising a metal powder with such a smallparticle size, sintering of the metal powder proceeds rapidly beforesintering of the ceramic powder used to form the ceramic laminatebegins, tending to cause structural defects such as delamination, owingto the difference of the shrinkage behaviors between the ceramic powderand the metal powder.

Japanese Unexamined Patent Application Publication 4-45835 discloses amethod for causing local hydrolysis with a metal alkoxide on the surfaceof a metal particle so as to form compact layers of minute metal oxideparticles such as titania or silica on the surface. Furthermore,Japanese Unexamined Patent Application Publication 11-45617 discloses amethod for coating particles of a base metal such as manganese ormanganese oxide. However, if an electroconductive paste comprising sucha metal powder is used for forming an electrode film, and the film isbaked at the same time with ceramic green sheets, the silica or the likecoating the metal powder diffuses into the ceramic and accelerates thegrain growth of the ceramic grains, with the result that such growth ofceramic grains may entail problems of interlayer delamination and crackgeneration.

Furthermore, Japanese Unexamined Patent Application Publication10-324906 discloses a method for manufacturing a nickel powder by aspray thermal decomposition method in which, for example, compositeoxides comprising lanthanum and nickel are deposited on the surface ofthe nickel powder. It discloses that contact between nickel metalparticles can be prevented when internal electrodes are formed with anelectroconductive paste comprising such a nickel powder, and thereforethe sintering temperature of the nickel powder can be shifted to ahigher level, with the result that a ceramic electronic component can beobtained which is prevented from delamination. However, sintering of thenickel according to this method proceeds rapidly once the coating filmmade of the composite oxides existing on the surface of the nickelpowder is destroyed during the baking step, with the result that thereappears a problem of rapid stress generation in the internal electrodeswhich will induce crack generation.

A metal powder gas phase manufacturing method such as the spray thermaldecomposition method also has a problem in that it is more expensive andmore complex than a liquid phase metal powder manufacturing method.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to solve theabove-described problems, and to provide a metal powder and amanufacturing method therefor wherein sintering of the metal powder isrestricted at a low temperature, the sintering initiation temperature isshifted to a higher level, and rapid sintering shrinkage is restricted,while grain growth of a ceramic is not accelerated.

For achieving the above-described object, the metal powder manufacturingmethod according to the present invention is characterized in that ametal powder is obtained from a mixture comprising a caustic alkali,hydrazine and/or hydrazine hydrate, a salt of electroconductive metal, arare earth metal salt and a main solvent, by depositing a rare earthmetal hydroxide derived from the above-described rare earth metal salt,and by reducing the above-described salt of electroconductive metal.

In particular, the metal powder manufacturing method according to thepresent invention preferably comprises the steps of: preparing areducing solution comprising the caustic alkali, the hydrazine and/orhydrazine hydrate and the main solvent; preparing a metal salt solutioncomprising the salt of electroconductive metal, the rare earth metalsalt and the main solvent; and mixing the reducing solution with themetal salt solution to form a metal powder by depositing a hydroxidederived from the rare earth metal salt and by reducing the salt ofelectroconductive metal.

The present invention is characterized in that the reduction reaction ofa salt of electroconductive metal and the deposition reaction of a rareearth metal hydroxide are performed simultaneously in one reaction stepso that a metal powder is recovered in which the metal obtained by thereduction of the salt of electroconductive metal and the deposited rareearth metal hydroxide exist in an intermingled state. Such a metalpowder is different from a metal powder in which minute metal oxideparticles or the like coat only the surface of the metal in that a rareearth metal hydroxide is intermingled with the metal powder, with theresult that not only are the metal particles formed by reducing the saltof electroconductive metal are prevented from contacting with each otherbut also undesirable migration of the metal particles can be prevented.Accordingly, rapid sintering of the metal powder can be prevented at thetime of baking an electroconductive paste comprising this metal powder.

It is noted that the state in which a rare earth metal hydroxide isintermingled with a metal powder is a state in which minute rare earthmetal hydroxide particles exist on the surface of as well as inside themetal powder thus obtained.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a multilayer ceramic electroniccomponent according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, the amount of the rare earth metal salt used,when converted to the amount of the rare earth metal, is preferably notless than about 0.1% by weight based on 100% by weight of the salt ofelectroconductive metal when converted to the amount of the metal. Whenthe amount is less than about 0.1% by weight, the above-described effectis hard to obtained.

Furthermore, the salt of electroconductive metal for use in the presentinvention can be reduced by hydrazine, and the metal can be deposited asa single metal or as an alloy by the reduction reaction. Furthermore,the metal thus reduced is required to show electroconductivity.

Such a salt of electroconductive metal is preferably a metal saltcomprising at least one of palladium, and copper and nickel, which arecommon base metals as electroconductive materials for anelectroconductive paste for forming electrodes of an electroniccomponent. However, there is no specific limitation of the salt ofelectroconductive metal. It is preferably well soluble in the chosenmain solvent, and, therefore, it is preferably a chloride, sulfate ornitrate.

Furthermore, the rare earth metal for use in the present invention isrequired to be able to exist as a hydroxide in the presence of a causticalkali. By virtue of this rare earth metal hydroxide existing in themetal powder, the sintering behavior of the metal powder is improved.Although there is no specific limitation of the rare earth metal salt,it is preferable that the salt is well soluble in the chosen mainsolvent, and, therefore, it is preferably a chloride, sulfate ornitrate. As the rare earth metal, it is preferable to use at least oneselected from the group consisting of lanthanum, cerium, neodymium,samarium, europium, gadolinium, terbium, dysprosium and ytterbium.

Furthermore, it is preferable that the caustic alkali for use in thepresent invention be at least one selected from the group consisting ofpotassium hydroxide, sodium hydroxide, calcium hydroxide, bariumhydroxide and ammonia. When hydrazine is used in reducing a metal salt,it is necessary to supply a hydroxide ion. In the metal powdermanufacturing method according to the present invention, the causticalkali acts as a source for supplying a hydroxide ion. It also has thefunction of depositing the rare earth metal salt as a hydroxide. It ismost preferable to use sodium hydroxide as the only caustic alkali.

The main solvent for use in the present invention is preferably a mixedsolvent of an alcohol and water. The alcohol is preferably at least onemonohydric alcohol such as methanol, ethanol and propanol. Throughreduction to a metal by selecting hydrazine as a reducing agent and analcohol as a main solvent, a metal powder having a particle size of notmore than about 100 nm can be obtained, and a ceramic electroniccomponent having thinner layers of internal electrodes can be realizedby using an electroconductive paste comprising such minute metalparticles. It is noted that by selecting a solvent comprising an alcoholas a main solvent, the solubility of an metal ion can be lowered incomparison with a case in which only water is used as a solvent, withthe result that the deposition rate of the metal can be raised and theparticle size of the deposited metal powder can be reduced. Accordingly,it is possible to control the particle size of a deposited metal powderby changing the concentration of the alcohol in the main solvent.

Preferable embodiments of the metal powder manufacturing methodaccording to the present invention are described below.

First, a caustic alkali is dissolved into a main solvent at a molarconcentration of from about 0.5 to 5.0 mol/L, using ethanol as the onlymain solvent and sodium hydroxide as the only caustic alkali.Furthermore, hydrazine or hydrazine hydrate is dissolved into thesolution so as to form a reducing solution in an amount ranging up toabout 15 times the amount which is stoichiometrically necessary forreducing the salt of electroconductive metal to be used.

Next, a chloride of electroconductive metal comprising at least oneselected from the group consisting of palladium, copper and nickel, inan amount of from about 0.01 to 1 mol/L, and a rare earth metal chloridecomprising at least one selected from the group consisting of lanthanum,cerium, neodymium, samarium, europium, gadolinium, terbium, dysprosiumand ytterbium, are dissolved into the main solvent to form a metal saltsolution. In a more specifically preferred embodiment, the amount of therare earth metal salt to be added, when converted to the amount of themetal, is not more than about 30% by weight based on 100% by weight ofthe salt of electroconductive metal to be added when converted to theamount of the metal. When the amount of the rare earth metal salt is notmore than about 30% by weight, the above-described effect is fullyachieved, and yet the equivalent series resistance of a ceramicelectronic component having internal electrodes formed with such anelectroconductive paste can be restricted low.

Next, the above-described reducing solution and the metal salt solutionare mixed together. Here, it is preferable that the temperatures of bothsolutions as well as that of the mixed solution be in the range of fromabout 20 to 60° C. When the temperature of the mixed solution, that is,reaction temperature exceeds about 20° C., the reduction reactionproceeds easily. When the temperature of the mixed solution does notexceed about 60° C., the concentration of the mixed solution remainsrelatively constant since ethanol or the main solvent does notevaporate. The reduction reaction proceeds in a high pH alkaline region.Since typically the pH of the reducing solution is about 13 and the pHof the metal salt solution is about 5, it is possible to make thereaction proceed while maintaining a high pH state if the metal saltsolution is added into the reducing solution dropwise. Therefore, it ispreferable as a procedure for the mixing to add the metal salt solutiondropwise into the reducing solution under agitation, although it is notnecessarily a limiting condition.

The metal which has been deposited by the reduction of the metal salt isseparated, recovered, washed with water, acetone or the like, and driedto provide a metal powder.

The metal powder thus obtained is in a state in which the metal powderis intermingled with a rare earth metal hydroxide, or more particularly,minute rare earth metal hydroxide particles having an average particlesize of from several nm to several tens of nm exist on the surface ofand inside the metal particles having an average particle size on theorder of from about 40 nm to 100 nm. Accordingly, not only mutualcontact between the metal particles but also undesirable migration ofthe metal particles can be prevented, and therefore rapid sintering ofthe metal powder can be prevented at the time of baking anelectroconductive paste comprising this metal powder, owing to the rareearth metal hydroxide existing both on the surface and on the inside.The rare earth metal hydroxide is preferably adsorbed on the wholesurface area of the metal powder.

The electroconductive paste according to the present invention comprisesa metal powder according to the above-described present invention and apasty carrier which is conventionally the combination of an organicvehicle and a solvent. Although there is no specific limitation to thetype and the content of the organic vehicle and the solvent, a preferredpaste is obtained, for example, by adding 40% by weight of an organicvehicle prepared by dissolving 10 parts by weight of an ethylcellulosetype resin as an organic binder into 90 parts by weight of terpineol asa solvent and 50% by weight of a metal powder according to theabove-described present invention, to 10% by weight of terpineol,followed by a dispersing and blending treatment with a three-roll mill.

One embodiment of a ceramic electronic component according to thepresent invention will be described in detail based on FIG. 1.Accordingly, a ceramic electronic component 1 is, for example, composedof a ceramic laminate 2, internal electrodes 3, external electrodes 4and plating films 5.

The ceramic laminate 2 is formed by baking a raw ceramic laminateobtained by laminating a plurality of ceramic layers 2 a prepared from adielectric material comprising BaTiO₃ as a main component.

The internal electrodes 3 are located between the ceramic layers 2 ainside the ceramic laminate 2. They are formed by providing printingwith an electroconductive paste onto the plurality of raw ceramic layers2 a, followed by baking a raw ceramic laminate which was formed bylaminating a plurality of the raw ceramic layers having the paste layersthereon. One edge of each of the internal electrodes 3 is formed so asto be exposed to one of two opposing edge surfaces of the ceramic layers2.

The external electrodes 4 are formed by applying an electroconductivepaste to the edge surfaces of the ceramic laminate 2 and baking so as tobe connected electrically and mechanically to either of the edges ofinternal electrodes 3 exposed on the edge surfaces of the ceramiclaminate 2.

The plating films 5 are composed, for example, of an electroless platingof Sn, Ni or the like, a solder plating, or other platings. They arecomposed of at least one layer respectively on the external electrodes4.

The material for a ceramic laminate 2 of a ceramic electronic componentaccording to the present invention is not limited to the above-describedembodiments. It may be formed, for example, of a different dielectricmaterial such as PbZrO₃, an insulating material, a magnetic material ora semiconductor material. Furthermore, the number of the layers ofinternal electrodes of a multilayer ceramic electronic componentaccording to the present invention is not limited to the number shown inthe above-described embodiments. There is no limitation to the number ofthe layers, and there may also be a case in which a multilayer ceramicelectronic component has no internal electrodes, i.e., the fine powderedmetal is used in the external electrode(s). Furthermore, the platingfilms 5 are not necessarily required. There is also no limitation to thenumber of the layers of the plating films 5.

EXAMPLES

Concrete examples of the present invention will be described below.However, the present invention is not limited by the examples below.

Example 1

First, 2 g of sodium hydroxide and 5 g of hydrazine hydrate weredissolved into 100 mL of ethanol to form a reducing solution.

Next, nickel chloride and lanthanum chloride were mixed together at theblending ratios described in the following Table 1. They were dissolvedinto 100 mL of ethanol to form the metal salt solutions of Samples 1 to8. Only nickel chloride was dissolved into 100 mL of ethanol in the sameway to form the metal salt solutions of Samples 9 and 10.

Next, the metal salt solutions of Samples 1 to 10 were added to thereducing solution dropwise with agitation of the reducing solution toform a series of mixed solutions, while keeping the reducing solutionand the sample metal salt solutions at 60° C., so that the nickelchloride was reduced to form a nickel powder in the mixed solutions.

Next, the nickel powders in the mixed solutions were separated,recovered, washed with pure water and then with acetone, followed bydrying in an oven to form nickel powders. These nickel powders wereapproximately spherical with a particle size of from 40 to 60 nm underobservation with a scanning electron microscope. From an X-raydiffraction measurement, the spectra of Ni and La(OH)₃ were found in thenickel powders obtained from the metal salt solutions of Samples 1 to 7,and a spectrum of Ni was found in the nickel powders obtained from themetal salt solutions of Samples 8 to 10.

Next, the nickel powder obtained from the Sample 10 metal salt solutionwas subjected to a surface treatment with silica. Accordingly, it wastreated according to the same method as described in Example 8 ofJapanese Unexamined Patent Application Publication 4-45835.Specifically, the nickel powder with water adsorbed on the surface wasdispersed by ultrasonic waves into an organic solvent prepared by addingsilicon ethoxide, isopropyl amine and isopropanol into hexane. It wasthen subjected to reaction at 25° C. for 48 hours under agitation with astirrer. The reaction product was washed with hexane, methyl alcohol orthe like, dried, and then was subjected to a heat treatment at 450° C.to obtain a nickel powder, the surface of which was coated withultrafine silica particles.

Next, the electroconductive paste Samples A to J were prepared by adding40% by weight of an organic vehicle formed by dissolving 10 parts byweight of an ethylcellulose type resin as an organic binder into 90parts by weight of terpineol as a solvent and 50% by weight of thenickel powders obtained from the metal salt solutions of Samples 1 to10, to 10% by weight of terpineol, followed by a dispersing and blendingtreatment with a three-roll mill.

Next, barium titanate (BaTiO₃) powder manufactured by a hydrolysismethod and having an average particle size (D₅₀) of 0.5 μm was preparedas a ceramic powder.

Next, an additive (sintering adjuvant) comprising 0.02 mol % of Dy, 0.02mol % of Mg, 0.02 mol % of Mn and 3 mol % of Si in a state of oxidepowders or carbonate powders is added to the barium titanate powder formixing. A polyvinylbutyral type binder and an organic solvent such asethanol were further added to the mixture, followed by wet blending witha ball mill. A ceramic slurry was thus prepared.

Next, ceramic green sheets with a thickness of 1.4 μm were formed usingthe ceramic slurry by adjusting the slit breadth of the doctor bladeaccording to a doctor blade method.

The ceramic green sheets thus obtained were subjected to printing withthe electroconductive paste Samples A to J to form internal electrodepatterns. Specifically, a plurality of ceramic green sheets havingelectrode films with a thickness after drying of 0.6 μm were obtained byapplying screen printing to the surfaces of a specific number of ceramicgreen sheets so that one edge of each internal electrode pattern wasexposed at one of the opposing edge surfaces of the ceramic laminatewhich were to be ultimately obtained, and the other edge stayed insidethe ceramic laminate, and by changing the thickness of the screenpatterns.

Next, a plurality of ceramic green sheets each with internal electrodepatterns formed with the electroconductive paste Samples A to J werelaminated, were unified by thermal pressing, and were then cut to aspecific size to provide raw ceramic laminates.

Next, they were heated in an N₂ atmosphere at 400° C. to burn off theorganic binder, and were held in a reducing atmosphere of H₂—N₂—H₂O gasat an oxygen partial pressure of 9×10⁻¹² MPa and in a temperature rangeup to a maximum baking temperature of 1,200° C. for three hours so as toform a ceramic laminate. The ceramic laminate after baking had 250dielectric ceramic layers, and the internal electrode area per ceramiclayer was 17.8×10⁻⁶ m².

The nickel powders obtained from the metal salt solutions of Samples 1to 10 were subjected to inductively coupled plasma-atomic emissionspectrometry (ICP-AES) for measuring composition for nickel, lanthanumand silicon. Their sintering behaviors (the initiation and terminationtemperatures of sintering shrinkage) were also investigated bythermomechanical analysis (TMA). The results are summarized in thefollowing Table 1.

TABLE 1 Nickel Lanthanum Lanthanum Sintering shrinkage (° C.) chloridechloride Silicon content Initiation Termination Temperature SampleNickel (g) Lanthanum (g) (g) ratio (%) temperature temperaturedifference 1 1.2 5.0 1.5   4.0  — 56    501 980 479 2 1.2 5.0 0.75  2.0  — 38    453 958 505 3 1.2 5.0 0.375  1.0  — 23    420 932 512 4 1.25.0 0.15   0.4  — 11.1   402 911 509 5 1.2 5.0 0.015  0.04  — 1.2  356825 469 6 1.2 5.0 0.0075  0.02  — 0.62  276 701 425 7 1.2 5.0 0.0015 0.004 — 0.12  252 680 428 8 1.2 5.0 0.00075 0.002 — 0.062 225 455 230 91.2 5.0 — — — — 221 452 231 10 1.2 5.0 — — 0.012 — 325 778 453

Regarding the ceramic laminates comprising internal electrodes formedwith the electroconductive paste Samples A to J, the delaminationoccurrence rates between the ceramic layers, the coverages (the coatedarea rates of the internal electrodes), the crack occurrence rates, theaverage grain sizes of the ceramic grains, and the ESRs (equivalentserial resistances) were measured. The results are summarized in thefollowing Table 2.

The delamination occurrence rates were determined as follows: a ceramiclaminate was cut along the surface in the direction perpendicular to thelengthwise direction; surfaces thus cut were polished and were subjectedto microscopic observation with the naked eye to count the number ofsamples in which interlayer delamination was observed. The number ofsamples in which interlayer delaminating was generated out of 100samples was determined as the rate.

Regarding coverage, surfaces of internal electrodes of ceramic laminateswere peeled away, pictures of the surfaces of the internal electrodeshaving pores were taken through a microscope, and they were subjected toa picture analysis as a quantitative analysis.

The crack occurrence rates were determined as follows: a ceramiclaminate embedded in a resin was cut along the surface in the directionperpendicular to the lengthwise direction; surfaces thus cut werepolished and were subjected to microscopic observation with the nakedeye to counting the number of samples in which cracks appeared insidethe internal electrodes. The number of samples in which cracks weregenerated out of 100 samples was determined as the rate.

The average grain sizes of the ceramic grains were determined asfollows: a ceramic laminate was cut along the cross-section in thedirection perpendicular to the lengthwise direction; surfaces thus cutwere polished and were subjected to a chemical etching treatment; andthey were subjected to observation under a scanning electron microscopefor determining the size.

Regarding the ESRs, an electroconductive thick film paste having Ag asan electroconductive component was applied to both edge surfaces of theceramic laminates at which the internal electrodes were exposed,followed by baking to form external electrodes. The ESRs were measuredon these electrodes.

TABLE 2 Crack Average Delamination occurrence grain Sam- occurrence rateCoverage rate size ESR ple (%) (%) (%) (μm) (W) A 1 0 60 0 0.5 524 B 2 075 0 0.5 495 C 3 0 88 0 0.5 142 D 4 0 92 0 0.5 137 E 5 0 89 0 0.5 125 F6 0 85 0 0.5 118 G 7 0 82 0 0.5 111 H 8 55 65 100 0.5 107 I 9 58 67 1000.5 103 J 10 75 85 60 3.8 107

As is evident from Table 1, it was found that the nickel powders(Samples 1 to 8) formed by adding lanthanum chloride to metal saltsolutions followed by reduction, had initiation and terminationtemperatures of sintering shrinkage which were shifted to higher levelsin comparison with the nickel powders (Samples 9 and 10) formed byreduction without adding lanthanum chloride. Furthermore, thetemperature differences between the initiation temperatures and thetermination temperatures of sintering shrinkage became largerapproximately in proportion to the amount of added lanthanum chloride,indicating that sintering of nickel powders proceeds more gently as theamount of lanthanum chloride is increased.

Furthermore, the nickel powder (Sample 10) the surface of which wascoated with ultrafine silica particles without addition of lanthanumchloride showed a shrinkage behavior wherein it had initiation andtermination temperatures of sintering shrinkage approximately in themiddle of those of the Sample 5 nickel powder and Sample 6 nickelpowder.

As is evident from Table 2, with the ceramic laminates formed using theelectroconductive paste Samples C to G having nickel powders (Samples 3to 7) which comprised 0.12 to 23% by weight of lanthanum, thedelamination occurrence rates were all 0%, the coverages were in therange of from 82 to 92%, and the crack occurrence rates were all 0%.

With the ceramic laminates having internal electrodes formed with theelectroconductive paste Samples A and B which comprised nickel powders(Samples 1 and 2) having more than about 30% by weight, or, to be morespecific, 38% by weight or 56% by weight of lanthanum in 100% by weightof a nickel powder, the ESRs tended to show rather high values of from495 to 524 Ω but the occurrence rates of delamination and cracks wereboth 0%.

In contrast, with the ceramic laminates formed with theelectroconductive paste Samples H to J which comprised nickel powders(Samples 8 to 10) on which less than about 0.1% by weight of lanthanumwas deposited, or, more specifically, only 0.062% by weight of lanthanumwas deposited or no lanthanum was deposited at all, the delaminationoccurrence rates were as high as 55 to 75% and the crack occurrencerates were as high as 60 to 100%.

The ceramic laminate manufactured using the electroconductive pasteSample J comprising the nickel powder (Sample 10) whose surface wascoated with ultrafine silica particles had a large average grain size of3.8 μm. This is considered to have been caused by diffusion of thesilica coating on the nickel powder into the ceramic layers at the timeof baking the raw ceramic laminate, which resulted in unusual ceramicgrain growth. It is supposed that such unusual ceramic grain growthcauses interlayer delamination and crack generation at the time of orafter baking of a ceramic laminate.

Furthermore, as is evident from Table 1 and 2, all of the nickel powdersobtained from the metal salt solutions of Samples 3 to 7 had temperaturedifferences between the initiation temperatures and the terminationtemperatures of sintering shrinkage of not less than 300° C., andinitiation temperatures of sintering shrinkage of not more than 420° C.It is understood that when the above-described temperature difference isless than 300° C., sintering of a nickel powder proceeds rapidly, withthe result that cracks tend to be generated easily in a ceramiclaminate.

Example 2

First, 2 g of sodium hydroxide and 5 g of hydrazine hydrate weredissolved into 100 mL of ethanol to form a reducing solution in the sameway as in Example 1.

Next, copper nitrate and samarium chloride were mixed together atblending ratios described in the following Table 3. They were dissolvedinto 100 mL of methanol to form the metal salt solutions of Samples 11to 18. Only copper nitrate was dissolved into 100 mL of ion exchangewater in the same way to form the metal salt solutions of Samples 19 and20.

Next, the metal salt solutions of Samples 11 to 20 were added to thereducing solution dropwise with agitation of the reducing solution toform mixed solutions respectively, while keeping the reducing solutionand the metal salt solutions of Samples 11 to 20 at 50° C., so that thecopper nitrate was reduced to form copper powder in each mixed solution.

Next, the copper powder in the mixed solutions were separated,recovered, washed with pure water and then with acetone, followed bydrying at room temperature to form copper powders. These copper powderswere approximately spherical with a particle size of from 70 to 90 nm inobservation under a scanning electron microscope. From an X-raydiffraction measurement, spectra of Cu and Sm(OH)₃ were found in thecopper powders obtained from the metal salt solutions of Samples 11 to17, and a spectrum of Cu was found in the copper powders obtained fromthe metal salt solutions of Samples 18 to 20.

Next, the copper powder obtained from Sample 20 metal salt solution wassubjected to a surface treatment with manganese. Accordingly, it wastreated according to the same method as described in the Example ofJapanese Unexamined Patent Application Publication 11-45617.Specifically, a copper powder with an average particle size of 80 nmwherein the particle surface had been subjected to an activationtreatment was put into a strongly acidic solution comprising manganesesulfate to form a reaction product by controlling the manganese ionconcentration while maintaining the pH at 3. The product was thensubjected to a heat treatment in an oxygen atmosphere to obtain a copperpowder the surface of which was coated with ultrafine manganeseparticles.

Next, the electroconductive paste Samples K to T were prepared by adding40% by weight of an organic vehicle formed by dissolving 10 parts byweight of an ethylcellulose type resin as an organic binder into 90parts by weight of terpineol as a solvent and 50% by weight of thecopper powders obtained from the metal salt solutions of Samples 11 to20, to 10% by weight of terpineol, followed by a dispersing and blendingtreatment with a three-roll mill.

Next, ceramic green sheets with a thickness of 1.4 μm were formed in thesame way as in Example 1. These ceramic green sheets were subjected toprinting with the electroconductive paste Samples K to T to forminternal electrode patterns. Specifically, a plurality of ceramic greensheets having electrode films with a thickness after drying of 0.6 μmwere obtained by applying screen printing to the surfaces of a specificnumber of ceramic green sheets so that one edge of each internalelectrode pattern was exposed to one of the opposing edge surfaces ofthe ceramic laminate which were to be ultimately obtained, and the otheredge stayed inside the ceramic laminate, and by changing the thicknessof the screen patterns.

Next, a plurality of ceramic green sheets each with internal electrodepatterns formed with the electroconductive paste Samples K to T thereonwere laminated, were unified by thermal pressing, and were then cut to aspecific size to provide raw ceramic laminates.

Next, they were heated in an N₂ atmosphere at 250° C. to burn off theorganic binder, and were held in a reducing atmosphere of H₂—N₂—H₂O gasat an oxygen partial pressure of 9×10⁻¹² MPa and in a temperature rangeup to a maximum baking temperature of 1,100° C. for two hours so as toform ceramic laminates. Each ceramic laminates after baking had 100dielectric ceramic layers and the internal electrode area per ceramiclayer was 15.8×10⁻⁶ m².

The copper powders obtained from the metal salt solutions of Samples 11to 20 were subjected to inductively coupled plasma-atomic emissionspectrometry (ICP-AES) for measuring composition for copper, samariumand manganese. Their sintering behaviors (the initiation and terminationtemperatures of sintering shrinkage) were also investigated applying athermomechanical analysis (TMA). The results are summarized in thefollowing Table 3.

TABLE 3 Copper Samarium Samarium Sintering shrinkage (° C.) nitratechloride Manganese content Initiation Termination Temperature SampleCopper (g) Samarium (g) (g) ratio (%) temperature temperature difference11 2.6 10.0 4.1   10.0   — 61    450 793 343 12 2.6 10.0 1.23   3.0  —31    426 763 337 13 2.6 10.0 0.82   2.0  — 24    398 735 337 14 2.610.0 0.41   1.0  — 13.6   367 715 348 15 2.6 10.0 0.041  0.1  — 1.6  311628 317 16 2.6 10.0 0.021  0.05  — 0.80  280 583 303 17 2.6 10.0 0.0041 0.01  — 0.16  221 522 301 18 2.6 10.0 0.00210 0.005 — 0.081 194 409 21519 2.6 10.0 — — — — 195 412 217 20 2.6 10.0 — — 0.030 — 235 498 263

Regarding the ceramic laminates comprising internal electrodes formedwith the electroconductive paste Samples K to T, the delaminationoccurrence rates between the ceramic layers, the coverages (the coatedarea rates of the internal electrodes), the crack occurrence rates, theaverage grain sizes of the ceramic grains, and the ESRs (equivalentserial resistances) were measured. The results are summarized in thefollowing Table 4.

The delamination occurrence rates, the coverages, the crack occurrencerates, the average grain sizes, and the ESR's were measured in the sameway as in Example 1.

TABLE 4 Crack Average Delamination occurrence grain Sam- occurrence rateCoverage rate size ESR ple (%) (%) (%) (μm) (W) K 11 0 56 0 0.5 562 L 120 72 0 0.5 513 M 13 0 85 0 0.5 139 N 14 0 91 0 0.5 132 O 15 0 88 0 0.5124 P 16 0 83 0 0.5 118 Q 17 0 80 0 0.5 112 R 18 91 53 100 0.5 108 S 1993 55 100 0.5 103 T 20 83 78 87 2.7 109

As is evident from Table 3, it was found that the copper powders(Samples 11 to 18) formed by adding samarium chloride to metal saltsolutions followed by reduction, had initiation and terminationtemperatures of sintering shrinkage which were shifted to higher levelsin comparison with the copper powder (Sample 19) formed by reductionwithout adding samarium chloride. Furthermore, the temperaturedifferences between the initiation temperatures and the terminationtemperatures of sintering shrinkage became larger approximately inproportion to the amount of added samarium chloride, indicating thatsintering of copper powders proceeds more gently as the amount of addedsamarium chloride is increased.

Furthermore, the copper powder (Sample 20) the surface of which wascoated with ultrafine manganese particles without addition of samariumchloride had an initiation temperature of sintering shrinkage which wassituated between those of the Sample 16 copper powder and Sample 17copper powder, while it had a termination temperature of sinteringshrinkage which was situated approximately in the middle of those of theSample 17 copper powder and Sample 18 copper powder. From this, it isunderstood that if a copper powder formed by reduction through additionof samarium chloride into a metal salt solution has the same initiationtemperature of sintering shrinkage as that of Sample 20 copper powder,the temperature difference between the initiation temperature and thetermination temperature of sintering shrinkage of Sample 20 copperpowder becomes smaller, since its termination temperature of sinteringshrinkage is lower, with the result that the sintering proceeds morerapidly than those of the copper powders of Samples 11 to 18.

Furthermore, as is evident from Table 4, with the ceramic laminatesformed using the electroconductive paste Samples M to Q which comprisedcopper powders on which 0.16 to 24% by weight of samarium was deposited,the delamination occurrence rates were all 0%, the coverages were in therange of from 80 to 91% and the crack occurrence rates were all 0%.

Furthermore, with the ceramic laminates having internal electrodesformed with the electroconductive paste Samples K and L which comprisedcopper powders (Samples 11 and 12) having more than about 30% by weight,or, to be more specific, 31% by weight or 61% by weight of samariumdeposited in 100% by weight of the copper powder, the ESRs tended toshow rather high values of from 513 to 562 Ω, but the occurrence ratesof delamination and cracks were both 0%.

In contrast, with the ceramic laminates formed with theelectroconductive paste Samples R to T which comprised copper powders(Samples 18 to 20) on which less than about 0.1% by weight of samariumwas deposited, or, to be more specific, only 0.081% by weight ofsamarium was deposited or no samarium was deposited at all, thedelamination occurrence rates were as high as 83 to 93% and the crackoccurrence rates were as high as 87 to 100%.

The ceramic laminate formed with the electroconductive paste Sample Tcomprising the copper powder (Sample 20), the surface of which wascoated with ultrafine manganese particles, had a large average grainsize of 2.7 μm. This is considered to have been caused by diffusion ofthe manganese coating on the copper powder into the ceramic layers atthe time of baking the raw ceramic laminate, which resulted in unusualceramic grain growth. Such unusual ceramic grain growth causesinterlayer delamination and crack generation at the time of or afterbaking of a ceramic laminate.

Furthermore, as is evident from Tables 3 and 4, all of the copperpowders obtained from the metal salt solutions of Samples 13 to 17 hadtemperature differences between the initiation temperatures and thetermination temperatures of sintering shrinkage of not less than 300°C., and initiation temperatures of sintering shrinkage of not more than420° C. It is understood that when the above-described temperaturedifference is lower than 300° C., sintering of a copper powder proceedsrapidly, with the result that cracks tend to be generated easily in theceramic laminates.

As described above, according to the metal powder manufacturing methodof the present invention, it is possible to provide a metal powderwherein sintering of the metal powder is restricted at a lowtemperature, the sintering initiation temperature is shifted to a highertemperature, and rapid sintering shrinkage is restricted, while ceramicgrain growth is not accelerated. Furthermore, it is possible to supplyan electroconductive paste providing such unique effects as well as amultilayer ceramic electronic component equipped with internalelectrodes having such effects.

What is claimed is:
 1. An electroconductive paste comprising a metalpowder comprising an electroconductive metal having rare earth metalhydroxide on the surface of and in the inside thereof, and a vehicle. 2.An electroconductive paste according to claim 1, wherein said rare earthmetal hydroxide is adsorbed on substantially the whole surface of saidelectroconductive metal powder.
 3. An electroconductive paste accordingto claim 1, wherein said electroconductive metal is at least one memberselected from the group consisting of palladium, copper and nickel. 4.An electroconductive paste according to claim 3, wherein the averageparticle size is about 40 to 100 nm.
 5. A multilayer ceramic electroniccomponent, comprising: a ceramic laminate having edge surfaces andcomprising a plurality of ceramic layers; a plurality of internalelectrodes, each of which is disposed between a pair of said ceramiclayers such that one edge of the electrode is exposed at one edgesurface of said laminate; and an external electrode electricallyconnected to an exposed edge of an internal electrode, wherein saidinternal electrodes comprise a metal powder comprising anelectroconductive metal having rare earth metal hydroxide on the surfaceof and in the inside thereof.
 6. A multilayer ceramic electroniccomponent according to claim 5, wherein said multilayer ceramicelectronic component is a multilayer ceramic capacitor.